In the past, it has been desirable to determine the positioning of moving parts within a machine. For example, it has been desirable to determine the position of pistons within hydraulic/pneumatic actuators. However, such machines often require that positioning of the piston be detected without actually touching the piston, as the piston is moving during operation. In a hydraulic/pneumatic machine where a pressure vessel contains a moving piston, the placement of a sensor that extends through the pressure vessel walls can lead to leakage and a loss of operating pressure. Such leakage and loss of operating pressure can lead to a significant loss in effective operating life and efficiency.
One type of positioning apparatus for determining the position of a moving part within a machine is described in U.S. Pat. No. 4,369,398 which discloses an apparatus for monitoring vibrating equipment. Hall-effect switches are used to detect movement of a magnet on vibrating equipment that can result in overstroke or understroke. A control circuit is operable responsive to detected overstroke from the overstroke Hall-effect switch to generate an alarm and/or shut down the vibrating equipment. However, a pendulum member is used to detect when vibrating equipment undergoes oscillatory motion having an excess of amplitude, and a control circuit is used to shut the equipment down when the vibration is greater than a predetermined normal range. Accordingly, such pendulum only indirectly measures overstroke of the vibrating equipment, and other external vibration sources can induce movement of the pendulum member.
Another type of positioning apparatus for determining the position of a moving part within a machine is described in U.S. Pat. No. 4,907,435, which discloses a Hall-effect proximity switch that is positioned to cooperate with a switching arm that is driven rotatably by movement of an adjusting valve. The Hall-effect proximity switch detects motion of a rotating machine component having a slot therein for enabling control of a hydraulic valve type of positioning apparatus for determining the position of a moving part within a machine. However, the switching arm is driven in rotation and does not provide an efficient solution for monitoring the movement of purely reciprocating machine components.
Yet another type of positioning apparatus for determining the position of a moving part within a machine is described in U.S. Pat. No. 4,857,842, which discloses a temperature compensated Hall-effect position sensor. Such sensor can be used with hydraulic and pneumatic actuators having a magnetic piston and a non-magnetic cylinder. A pair of Hall-effect sensors are mounted adjacent a permanent magnet positioned on an outside of a hydraulic cylinder. The sensors are positioned upside-down relative to one another such that they perceive equal and opposite magnetic fields. Output signals are amplified and inverted, then added together. Such summing process cancels out any temperature-induced variations in the voltage output signals. As the piston approaches the position sensor, the magnetic field at the sensors rises from magnetic piston material forming a flux path between the magnet and the Hall-effect sensors. Hence, arrival of the piston at the piston sensor location can be determined. However, the cylinder must be non-magnetic. Furthermore, two separate Hall devices are needed in order to compensate for temperature effects. Even furthermore, a comparator is required for controlling operation of an external device depending on the position of an object with respect to the Hall-effect devices.
A similar problem of detecting and controlling moving member displacement amplitude is encountered with axially reciprocating displacers and pistons in power conversion machinery, such as Stirling cycle machines. However, a typical Stirling cycle machine includes a pressure vessel that houses a reciprocating displacer and a reciprocating piston and contains a thermodynamic working gas. A typical displacer forms a piston-type device that is movably carried within the housing. Reciprocating movement of the displacer within a chamber of the housing transfers working fluid between the front and back sides of the displacer, causing a thermodynamic transformation therebetween. Movement of the displacer occurs between a compression space, having a temperature somewhat above ambient, and an expansion space, having a low temperature (when configured in a cooler) or high temperature (when configured in an engine).
When configured as a Stirling cryocooler, an end portion of a reciprocating displacer forms a drive area in fluid contact with the compression space. The displacer end portion slidably extends through a bore in the housing in fluid communication with a compression space of a linear drive motor. The drive motor has a driving piston that operates on working gas in the compression chamber. The working gas then directly works on the displacer to produce motion. Hence, the driving piston and displacer form a free-piston machine, cooperating solely by action of the working fluid. A clearance seal is typically provided between the displacer end portion and the housing bore by maintaining an accurate reciprocating motion of the displacer and by providing an accurate relative sizing of the bore in the housing with the working piston and displacer end portion. The expansion space draws heat from a surrounding cold head, imparting cooling there along. The same construction can form a Stirling engine, by simply imparting heat to the cold head, causing the displacer to reciprocate, and moving the linear drive motor (which now operates as a linear alternator) to produce electric power.
For the case of a Stirling cycle machine, there exists a need to accurately monitor the position of both the linear drive motor piston and the displacer piston. Furthermore, there exists a need to more accurately control moving member displacement amplitude in Stirling cycle machines.
According to one construction technique used by Applicant, a displacer is supported within a chamber of a pressure vessel housing in a sprung configuration for Stirling cycle power conversion machinery. The sprung configuration includes a pair of flexural bearing assemblies that are used to accurately position a reciprocating member in a housing with respect to a clearance seal. Details of one such construction are disclosed in Applicant's U.S. Pat. No. 5,642,618. This U.S. Pat. No. 5,642,618 is herein incorporated by reference. However, further improvements are needed to enhance the monitoring and control of moving parts within such closed-cycle thermodynamic machines.
Therefore, there is a need to provide an improved moving member detector and control system for a Stirling cycle machine. More particularly, there exists a need to provide for a moving member detector that accurately and economically detects moving members within a pressure vessel containing thermodynamic working gas in an accurate, relatively efficient, and cost-effective manner. Even furthermore, there is a need to control movement of moving members within such a closed-cycle thermodynamic machine based upon detected positioning of the moving members and/or operating parameters generated by the thermodynamic machine. For example, there exists a need to provide for a control system for a Stirling cycle cryocooler wherein a realized temperature at a cold head is utilized to regulate operation of the cryocooler. The present invention also arose from an effort to develop such an improved construction in a simplified, economical, and cost effective manner.